From experience, "head clearance" as defined above, starts to become an issue at about 0.012" in a 308 (7mm-08). A lot depends on the action stiffness as well. A Savage 99, in 308 Win, will not tolerate as much "head clearance" as a Savage 12. The Savage 99 is a bit more "springy" than a bolt action rifle. In the Savage 12, @ 0.008", most cases will come out properly fire formed, some will not. I was using 1F that may have been "many"F, and they were not fired in my rifle.
Remember the rule, "only use brass that was fired in the rifle you are reloading for". Some view it as a guideline, others ignore it. In your situation, you are reforming so you are OK, if you are comparing the reformed case to a 7mm-08 that has been fired in your rifle.
The phenomenon you describe occurs because the original shoulder is being pushed back by reformed neck, not the die. The same thing can occur if you crimp too much, either the neck bulges of the shoulder gets pushed back a bit.
Another critical dimension to check is neck diameter of your loaded ammunition. The necks may need to be turned, reduced in thickness.
Reformed brass, even when going from 308 > 7mm-08, may need to be fireformed with a somewhat reduced load first, then FL sized again.
Because you are using IVI brass, I'd advise against using it as a candidate for reforming in the first place, and further I wouldn't use it for reloading the necked down version. It seems to do better when necked up to 338 Federal or 358 Winchester.
Quite a few people will tell you otherwise, and I've gotten away with doing it, and I've been caught and punished for it as well.
IMHO, Lapua brass is the cheapest and best way to go, next would be factory ammo that has been fired in your rifle. Other brands of new brass will work, just expect to have more culls when weight sorting.
7mm-08 ammo is still produced, readily available, as is new brass.
Search for articles by Tubbs or other long range shooters.
